Probing Gamma-Ray Propagation at Very-High Energies with H.E.S.S. Observations of M87

The radio galaxy Messier 87 has an active galactic nucleus, from which observations have been made of very high energy (E > 100 GeV) 𝛾 -rays up to a few 10s of TeV using the H.E.S.S. telescope array. During monitoring campaigns and target of opportunity observations, several high flux states could be identified. The high photon statistics obtained during these states allow us to probe phenomena affecting VHE 𝛾 -ray propagation across cosmic distances. Interaction of these photons with the Extragalactic Background Light (EBL) causes an attenuation, leading to an observed cut-off in the spectra of very high energy sources. Observations of M87 can be utilized to make measurements of the local EBL, specifically in the far infrared range. Furthermore, M87 is located at the heart of the nearby Virgo galaxy cluster located 18 Mpc away which is host to an intra-cluster magnetic field calculated at 34.2 𝜇 G. This allows us the opportunity to probe for effects of the mixing of light pseudo-scalar Axion Like Particles (ALPs) with photon states. These dark matter candidates are predicted to exhibit an oscillation effect in the presence of magnetic fields, oscillating between photon and ALP states. This energy-dependent effect would lead to the disappearance of photons, which would result in measurable “wiggles" in the source’s spectra. We find that the